In the last decade many research groups have been investigating synthesis and properties of hybrid core-shell nano-particles and inorganic hollow nano-particles. The vast number of scientific and patent publications resulting from this work has been reviewed in several articles; including Adv. Mater. 2010, 22, 1182-1195; Nano Today 2009, 4, 494-507; J. Coll. Interf. Sci. 2009, 335, 1-10; and Adv. Mater. 2008, 20, 3987-4019. Various different synthetic strategies for making hollow inorganic particles can be distinguished. A typical approach applies a micro- or nano-sized organic structure in a solvent system as a template or scaffold for forming an inorganic oxide outer layer (also referred to as coating with inorganic oxide), resulting in hybrid core-shell (or coated) nano-particles as intermediate product. Shell layers comprising silica are generally made with a sol-gel process based on the so-called Stöber method, wherein a tetra-alkoxy silane is hydrolysed and condensed in water/alcohol mixtures containing ammonia.
Within the context of this application the template is considered to substantially consists of organic compounds, like polymers with a back-bone based on C, and optionally O, N, S, etc., such that the template or core can be substantially removed from the core-shell particles, for example by dissolution or thermal degradation and evaporation (pyrolysis or calcination). By selectively removing the organic core hollow particles can be obtained; either as such or formed in situ as part of a composition or product. In many cases the inorganic shell formed comprises, or substantially consists of (modified-) silica. Use of silica as a coating or shell material offers various advantages; like versatility in synthetic routes, tailoring of properties of the shell layer, and relatively low cost.
Core-shell and/or hollow nano-particles can be used for many different applications. Their capacity to encapsulate a compound, like a pharmaceutical agent or a catalyst, can be applied in uses ranging from chemical processes to drug delivery and biomedical applications. Characteristics like low density, high surface area, etc. make such nano-particles attractive as catalyst support, as filler material, and as component in optical coatings, like anti-reflection (AR) layers.
Specific properties and compositional characteristics of such nano-particles are depending on the targeted application. For use in AR coatings, for example, particle size of the particles is an important parameter. A single layer AR coating on a transparent substrate typically should have a refractive index between the refractive indices of the substrate and air, in order to reduce the amount of light reflected at the substrate-air interface and increase the amount of light transmitted through the substrate. For example, in case of a glass with refractive index 1.5 the AR layer typically has a refractive index of about 1.2-1.3, and ideally of about 1.22. A porous silica or other inorganic oxide layer having sufficiently high porosity can provide such a low refractive index and function as AR coating, if its layer thickness is about ¼ of the wavelength of the light; meaning that in the relevant wavelength range of 300-800 nm the thickness preferably is in the range 70-200 nm. This of course means that the size and geometry of pores in such coating should be compatible with said layer thickness.
A porous AR coating layer can be made by different approaches using various pore forming agents; of which using porous or hollow particles in a binder or matrix material represents an elegant way to control porosity level and pore sizes. A coating composition comprising pre-fabricated hollow inorganic nano-particles and a matrix-forming binder based on silica precursors is for example described in EP1447433. Many documents address using organic-inorganic core-shell nano-particles in an AR coating composition. During formation of a cured coating layer the organic core can for example be removed by thermal degradation and evaporation or by selective dissolution, also depending on the type of matrix or binder used. WO2008028640 represents an example of a publication applying such hybrid nano-particles with a sacrificial organic core. In this document cationic polymer micelles or cationically stabilised polymer latex particles are used as organic template for making hybrid core-shell nano-particles and AR coating compositions.
Optimum pore size of such AR coating is not only depending on the coating layer thickness as mentioned above, but also on other desired performance characteristics. For example, if the layer contains very small pores, this may result in non-reversible moisture up-take via capillary condensation; affecting refractive index and making the coating layer more prone to fouling with other components. Such effects have been reported for so-called meso-porous silica having pores in the range 1-20 nm. Too large pores on the other hand may deteriorate mechanical strength of the coating, e.g. reduced abrasion resistance, or result in haze. Ideally, pore size can be controlled and set within the 20-200 nm range, to optimize various properties of the coating.
In case hybrid core-shell nano-particles are applied for making AR coatings, size and shape of the particles are highly dependent on the organic template used. The template is therefore in some publications also referred to as structure-directing agent. In above cited publications and references cited therein, numerous organic compounds, like surfactants and (block-)copolymers that can form colloidal aggregates, have been described as templates. Examples of colloidal aggregates include micelles, worms, vesicles, lattices, and mixed structures.
Nevertheless, there remains a need in industry for a method of making hybrid organic-inorganic core-shell nano-particles with improved control of template particle size and structure, as a tool to improve their performance in use, for example in an AR coating composition comprising core-shell nano-particles thus obtained.
It is therefore an objective of the present invention to provide such an improved method.
The solution to above problem is achieved by providing the method as described herein below and as characterized in the claims.
Accordingly, the present invention provides a method of making hybrid organic-inorganic core-shell nano-particles, comprising the steps of    a) providing colloidal organic particles as a template;    b) adding at least one inorganic oxide precursor; and    c) forming an inorganic shell layer from the precursor on the template to result in core-shell nano-particles,wherein the template comprises a synthetic polyampholyte, or stated differently,wherein the colloidal organic particles are based on a synthetic polyampholyte.
Within the context of the present application a polyampholyte is defined as an ampholytic or amphoteric copolymer, i.e. a synthetic (or man-made) copolymer or polyelectrolyte comprising at least one comonomer having a positively charged group and at least one comonomer having a negatively charged group. The polyampholyte thus comprises opposite charges on different pending groups. A copolymer comprising different charges on the same pending group represents a special type generally referred to as a zwitterionic polymer.
A comonomer having a positively or negatively charged group is understood to include a comonomer having a functional group that can be easily ionised, like carboxylic acid groups or tertiary amine groups by changing pH of the solvent system. Stated otherwise, a polyampholyte is a copolymer or polyelectrolyte containing both cationic and anionic groups, and/or their corresponding ionisable groups, and having a net charge under the conditions applied. Some authors define polyampholytes as charged polymers carrying both basic and acidic groups. The polyampholyte can have a positive or a negative net charge, depending on molecular composition and conditions; charge can for example be determined by measuring its zeta-potential (in solution/dispersion). A copolymer with a positive net charge will be called a cationic polyampholyte; negatively net charged copolymers will be referred to as being anionic polyampholytes.
With the method of the invention it is found possible to make colloidal organic particles or aggregates from a synthetic polyampholyte, the colloidal particles having an average particle size in the range of 10 to 300 nm. This particle size can be controlled by the comonomer composition of the polyampholyte, and/or by selecting conditions like temperature, pH, salt concentration, and solvent composition. Selecting and varying conditions to set particle dimensions enables one to make dispersions of colloidal organic particles and subsequently core-shell nano-particles with different particle size starting from one polyampholyte. A further advantage of the present method is that the dispersion of core-shell nano-particles obtained is very stable under different conditions; increasing its shelf life or storage time, and allowing for example altering its concentration and solvent system, making the dispersion suitable for various different application requirements.
With the method of the invention core-shell, and optionally porous or hollow particles can be obtained, which can advantageously be applied in different uses; including encapsulating of functional compounds and ingredients, as filler material in compositions for low-weight or isolating materials, or as component for lowering refractive index of a material like a coating, or for a coating with low gloss (surface roughness).
In WO2001/80823A2 and US2002/0064541A1 compositions comprising core-shell microcapsules for therapeutic or cosmetic use are described. The microcapsules of preferably 8-50 μm diameter are typically prepared by providing a shell comprised of an inorganic polymer around emulsified particles of active ingredient to be encapsulated. In making said emulsified core particles a wetting agent may be used, which can be a surfactant or polymeric surfactant. The polymeric surfactant can be anionic, cationic, amphoteric or non-ionic; and can be a hydrocarbon or a silicone polymer. Within long lists of suitable silicone polymers a silicone amphoteric polymer is mentioned, but the documents do not provide further information and are silent on potential effects of using such non-organic surfactant.
A further specific advantage of the invention is that a coating composition comprising the core-shell particles can be made into a coating with AR properties on a substrate via a thermal treatment at relatively low temperatures compatible with a plastic substrate, as well as at high curing temperature compatible with glass processing.
In the method according to the invention the synthetic polyampholyte used as template particle is an organic copolymer, comprising at least one monomer unit having a cationic charge, at least one monomer unit having an anionic charge, and optionally at least one neutral or non-ionic comonomer. The polymer may be a random, but also a block copolymer. The polyampholyte can be a condensation polymer, like a polyester, polyamide, polyurethane and the like; or an addition polymer, comprising styrenic, acrylic, methacrylic, olefinic, and/or vinylic comonomers. Within the context of this application all these monomers are together referred to as ethylenically unsaturated monomers or vinyl monomers; that is including methacrylates which comprise a methyl-vinyl group. Acrylic and methacrylic compounds are together typically referred to as (meth)acrylic monomers in the art. Preferably, the polyampholyte used in the method according to the invention is an addition polymer, which can advantageously be made using various known polymerisation techniques from a great number of suitable monomers; offering a wide range of compositions for the polyampholyte.
Preparation of such ampholytic addition copolymers for use in the method according to the invention is known from prior art, e.g from U.S. Pat. No. 4,749,762 and a number of documents cited therein. More specifically, U.S. Pat. No. 4,749,762 describes two alternative routes for making polyampholytes from (meth)acrylic monomers. In a one process acrylic acid, N,N-dimethylamine ethylmethacrylate (DMAEMA) or N,N-diethylamine ethylmethacrylate (DEAEMA), and optionally an alkyl(meth)acrylate are polymerised in solution in the presence of a strong acid, during which the amine groups are protonated. Alternatively, such mixture of comonomers—but comprising the methyl ester of acrylic acid—is (emulsion) polymerised, followed by selectively hydrolysing the acrylate ester comonomer (which is much faster than hydrolysis of methacrylate esters).
Synthesis of polyampholytes from various ethylenically unsaturated monomers is also described in U.S. Pat. No. 6,361,768 and references cited therein. Typically a radical polymerisation is performed in an organic solvent, and optionally surfactants are present to prevent agglomeration of copolymer formed.
In EP2178927 a dispersion of a cationic ampholytic copolymer is made by first copolymerising a mixture of monomers, for example methyl methacrylate (MMA), DMAEMA and methacrylic acid (MAA), in bulk or solution; followed by dispersing the copolymer in an aqueous medium (and neutralising non-ionic amine functional groups before or during dispersion).
In the method according to the invention the polyampholyte is preferably a copolymer obtained from                at least one cationic or basic monomer (M1), including compounds with a pending group that can combine with a proton; like monomers with a tertiary amine group;        at least one anionic or acidic monomer (M2), including compounds with a pending group that can yield a proton; like monomers containing carboxylic acid groups;        at least one neutral or non-ionic monomer (M3); preferably a non-water soluble or hydrophobic comonomer; and        optionally at least one cross-linking monomer (M4).        
The ionic comonomers M1 and M2 will increase solubility and dispersability of the copolymer in an aqueous system; whereas presence of non-ionic monomer units M3 may reduce solubility, but promote forming aggregates. Too high an amount of M3 may result in insolubility and/or precipitation of the copolymer. The type and amount of M3 is thus preferably chosen such that the polymer can still be dispersed in an aqueous medium into colloidal particles, M3 units promoting self-association by non-polar or hydrophobic interaction. Optionally, the copolymer may comprise a small amount of di- or polyfunctional monomer M4, which will induce a level of cross-linking that may further stabilize the colloidal particles formed. Typically such random copolymers can already form suitable aggregates in an aqueous medium; thus omitting the need to use more complex synthetic routes of making block copolymers.